1. Field of the Invention
This invention relates to molded case circuit breakers and more particularly to improved joints for fastening a form wound shunt, wound from a continuous strip of an electrical conductor, to a contact arm subassembly.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and relatively high level short circuit. An overload condition is about 200-300% of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000% or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers include at least one pair of separable contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation the contacts may be opened by an operating mechanism or by a magnetic repulsion member. The magnetic repulsion member causes the contacts to separate under relatively high level short circuit conditions. More particularly, magnetic repulsion members are connected between a pivotally mounted contact arm assembly and a stationary conductor assembly. Each magnetic repulsion member is a generally V-shaped member defining two depending legs, form wound from laminated strips of copper. During high level short circuit conditions, magnetic repulsion forces are generated between the depending legs of the magnetic repulsion members as a result of the current flowing therethrough which, in turn, causes the pivotally mounted contact arm assembly to blow open.
The pivotally mounted contact arm assembly is formed as a laminated assembly consisting of a plurality of contact arm subassemblies. Each subassembly includes a contact arm portion, which carries a movable main or arcing contact, and a stationary conductor portion, normally fastened to the circuit breaker frame. The free ends of the flexible shunt or magnetic repulsion member are disposed in keyholes formed in the stationary conductor portion and the contact arm portions forming a pair of joints.
In known contact arm subassemblies, for example, as disclosed in U.S. Pat. No. 4,891,618, and assigned to the same assignee as the present invention, the joints are soldered for mechanical strength and for improving the electrical conductivity of the joints. In order to prevent wicking of the solder into the copper laminations, the joints are staked before the soldering process.
Such a process for making such joints can be relatively expensive from a manufacturer's standpoint because it involves two operations for the joint: staking and soldering. Also in such a process the electrical resistance of the joints formed is dependent, in part, upon the contact between the free ends of the shunt and the keyholes. Thus, any tolerances in the components could cause irregular contact between the surfaces thereby increasing the electrical resistance of the joint; an undesirable condition.